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MMAP(2)                   FreeBSD System Calls Manual                  MMAP(2)

     mmap - allocate memory, or map files or devices into memory

     Standard C Library (libc, -lc)

     #include <sys/mman.h>

     void *
     mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

     The mmap() system call causes the pages starting at addr and continuing
     for at most len bytes to be mapped from the object described by fd,
     starting at byte offset offset.  If len is not a multiple of the
     pagesize, the mapped region may extend past the specified range.  Any
     such extension beyond the end of the mapped object will be zero-filled.

     If addr is non-zero, it is used as a hint to the system.  (As a
     convenience to the system, the actual address of the region may differ
     from the address supplied.)  If addr is zero, an address will be selected
     by the system.  The actual starting address of the region is returned.  A
     successful mmap deletes any previous mapping in the allocated address

     The protections (region accessibility) are specified in the prot argument
     by or'ing the following values:

     PROT_NONE   Pages may not be accessed.
     PROT_READ   Pages may be read.
     PROT_WRITE  Pages may be written.
     PROT_EXEC   Pages may be executed.

     The flags argument specifies the type of the mapped object, mapping
     options and whether modifications made to the mapped copy of the page are
     private to the process or are to be shared with other references.
     Sharing, mapping type and options are specified in the flags argument by
     or'ing the following values:

     MAP_ANON          Map anonymous memory not associated with any specific
                       file.  The file descriptor used for creating MAP_ANON
                       must be -1.  The offset argument is ignored.

     MAP_FIXED         Do not permit the system to select a different address
                       than the one specified.  If the specified address
                       cannot be used, mmap() will fail.  If MAP_FIXED is
                       specified, addr must be a multiple of the pagesize.  If
                       a MAP_FIXED request is successful, the mapping
                       established by mmap() replaces any previous mappings
                       for the process' pages in the range from addr to addr +
                       len.  Use of this option is discouraged.

     MAP_HASSEMAPHORE  Notify the kernel that the region may contain
                       semaphores and that special handling may be necessary.

     MAP_INHERIT       This flag never operated as advertised and is no longer
                       supported.  Please refer to minherit(2) for further

     MAP_NOCORE        Region is not included in a core file.

     MAP_NOSYNC        Causes data dirtied via this VM map to be flushed to
                       physical media only when necessary (usually by the
                       pager) rather than gratuitously.  Typically this
                       prevents the update daemons from flushing pages dirtied
                       through such maps and thus allows efficient sharing of
                       memory across unassociated processes using a file-
                       backed shared memory map.  Without this option any VM
                       pages you dirty may be flushed to disk every so often
                       (every 30-60 seconds usually) which can create
                       performance problems if you do not need that to occur
                       (such as when you are using shared file-backed mmap
                       regions for IPC purposes).  Note that VM/file system
                       coherency is maintained whether you use MAP_NOSYNC or
                       not.  This option is not portable across UNIX platforms
                       (yet), though some may implement the same behavior by

                       WARNING!  Extending a file with ftruncate(2), thus
                       creating a big hole, and then filling the hole by
                       modifying a shared mmap() can lead to severe file
                       fragmentation.  In order to avoid such fragmentation
                       you should always pre-allocate the file's backing store
                       by write()ing zero's into the newly extended area prior
                       to modifying the area via your mmap().  The
                       fragmentation problem is especially sensitive to
                       MAP_NOSYNC pages, because pages may be flushed to disk
                       in a totally random order.

                       The same applies when using MAP_NOSYNC to implement a
                       file-based shared memory store.  It is recommended that
                       you create the backing store by write()ing zero's to
                       the backing file rather than ftruncate()ing it.  You
                       can test file fragmentation by observing the KB/t
                       (kilobytes per transfer) results from an ``iostat 1''
                       while reading a large file sequentially, e.g. using
                       ``dd if=filename of=/dev/null bs=32k''.

                       The fsync(2) system call will flush all dirty data and
                       metadata associated with a file, including dirty NOSYNC
                       VM data, to physical media.  The sync(8) command and
                       sync(2) system call generally do not flush dirty NOSYNC
                       VM data.  The msync(2) system call is obsolete since
                       BSD implements a coherent file system buffer cache.
                       However, it may be used to associate dirty VM pages
                       with file system buffers and thus cause them to be
                       flushed to physical media sooner rather than later.

     MAP_PRIVATE       Modifications are private.

     MAP_SHARED        Modifications are shared.

     MAP_STACK         MAP_STACK implies MAP_ANON, and offset of 0.  The fd
                       argument must be -1 and prot must include at least
                       PROT_READ and PROT_WRITE.  This option creates a memory
                       region that grows to at most len bytes in size,
                       starting from the stack top and growing down.  The
                       stack top is the starting address returned by the call,
                       plus len bytes.  The bottom of the stack at maximum
                       growth is the starting address returned by the call.

     The close(2) system call does not unmap pages, see munmap(2) for further

     The current design does not allow a process to specify the location of
     swap space.  In the future we may define an additional mapping type,
     MAP_SWAP, in which the file descriptor argument specifies a file or
     device to which swapping should be done.

     Upon successful completion, mmap() returns a pointer to the mapped
     region.  Otherwise, a value of MAP_FAILED is returned and errno is set to
     indicate the error.

     The mmap() system call will fail if:

     [EACCES]           The flag PROT_READ was specified as part of the prot
                        argument and fd was not open for reading.  The flags
                        MAP_SHARED and PROT_WRITE were specified as part of
                        the flags and prot argument and fd was not open for

     [EBADF]            The fd argument is not a valid open file descriptor.

     [EINVAL]           MAP_FIXED was specified and the addr argument was not
                        page aligned, or part of the desired address space
                        resides out of the valid address space for a user

     [EINVAL]           The len argument was negative.

     [EINVAL]           MAP_ANON was specified and the fd argument was not -1.

     [EINVAL]           MAP_ANON has not been specified and fd did not
                        reference a regular or character special file.

     [EINVAL]           The offset argument was not page-aligned.  (See BUGS

     [ENOMEM]           MAP_FIXED was specified and the addr argument was not
                        available.  MAP_ANON was specified and insufficient
                        memory was available.  The system has reached the per-
                        process mmap limit specified in the vm.max_proc_mmap

     madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
     munlock(2), munmap(2), getpagesize(3), make.conf(5)

     The len argument is limited to 2GB.  Mmapping slightly more than 2GB does
     not work, but it is possible to map a window of size (filesize % 2GB) for
     file sizes of slightly less than 2G, 4GB, 6GB and 8GB.

     The limit is imposed for a variety of reasons.  Most of them have to do
     with FreeBSD not wanting to use 64 bit offsets in the VM system due to
     the extreme performance penalty.  So FreeBSD uses 32bit page indexes and
     this gives FreeBSD a maximum of 8TB filesizes.  It is actually bugs in
     the file system code that causes the limit to be further restricted to
     1TB (loss of precision when doing blockno calculations).

     Another reason for the 2GB limit is that file system metadata can reside
     at negative offsets.

     Note that an attempt to mmap() zero bytes has no effect and succeeds,
     while an attempt to munmap() zero bytes will return [EINVAL].

FreeBSD 11.0-PRERELEASE        November 17, 2001       FreeBSD 11.0-PRERELEASE


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